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Many factories dispose of chemical waste by dumping the waste into lakes, rivers or the ground. The pesticides that farmers use to eliminate insects can also reach surface water and groundwater in significant quantities. Underground storage tanks for gasoline and other liquids can directly contaminate groundwater. Salt placed on icy streets during winter also pollutes water, even though salt is not especially dangerous to people’s health.

Other sources of water pollutants include sewage and wastewater, septic tanks, and ocean and marine dumping. Soluble waste, or sewage, and wastewater come from the water people use at home and in public places such as schools and hospitals. The sewage often enters and pollutes various water bodies. The liquid content of septic tanks is typically drained out and dumped into a land drainage system. However, it sometimes goes into the soil and bodies of water. Waste materials, such as plastic and rubber, are also sometimes thrown into the sea.

The concentration of pollutants usually decreases as the pollutants become slowly diluted. However, this is a lengthy process. Even though the water is deemed safe to use when the pollutant concentration reaches a low level, the presence of pollutants is not entirely eliminated.